The invention relates to a method and apparatus for measuring the position of an interface between different materials in a storage tank or vessel.
There are many devices which are presently available for measuring the interface positions between two or more different materials. Such devices may, for example, be used for determining the level of a material (for example liquid) present in a (storage) vessel or container. Various techniques are known to provide such level or interface position measurements. An example of such a known technique is the use of microwaves. According to this known technique high-frequency electro-magnetic waves are directed toward the liquid level in the tank, where they are reflected from the liquid surface. The propagation time in the gas above the liquid gives a level-indication. This technique is virtually independent of the gas and the nature of the liquid, but it is rather costly. Further, undesirable spurious reflections from the vessel walls often create detection problems. Further, optical rangefinders exist which employ one or more discrete frequencies and measure the phase of the modulation of the reflected light. However, measuring phases and phase differences may provide technical difficulties and a good accuracy is not always obtained.
It is therefore an object of the invention to provide a method and apparatus for measuring the level of a material or the position of an interface between different materials, which is accurate and cheap and provides results, which are not affected by changes in the physical properties of the liquid.
It is another object of the invention to provide a method and apparatus for determining the level in a tank, involving radio frequency phase measurement, which is capable of using electrical power levels consistent with intrinsically safe operation.